Environmental protection standards require increasingly lower residual sulfur contents in combustibles and fuels, creating a supplemental need for hydrogen and requiring an increase in the treatment capacity of existing reactors for the hydrodesulfurization of hydrocarbons (for example of gasoline).
One of the solutions is to increase the partial pressure of hydrogen by purifying the hydrogen from conventional sources, particularly that from catalytic reformers.
The purification by cryogenic means, compared to other processes, is economically interesting when the capacity to purify hydrogen is great and is characterized by a final hydrogen purity of about 98 mol %, a high recovery output, about 96%, and a low energy consumption.
The cold requirement of this type of process is essentially ensured by the free expansion of impurities, principally constituted by hydrocarbons. For a catalytic reforming gas, whose composition conventionally is
______________________________________ H2 84 mol % CH4 6 mol % C2H6 5 mol % C3H8 3 mol % C4H10 2 mol % ______________________________________
the methane content is insufficient to ensure by free expansion the refrigeration power necessary for the coldest portion of the heat exchange line, and an expansion turbine of low power must therefore be provided to supply the necessary supplement.
Given the low power generated by the expansion of a small hydrogen flow of high purity whose molecular weight is about 2 kg/kmole, the use of turbines with gas bearings, permitting very high rotation speeds, is particularly advantageous.
The operation of turbines with gas bearings implies the use of so-called "utility gases", taken from the flow of purified hydrogen, to ensure the two conventional functions as follows:
on the one hand, a low flow at ambient temperature, so-called seal gas, is injected within the labyrinthine seal, provided between the rotor and the bearings, at a pressure slightly less than the interstitial pressure generated behind the turbine rotor by the escape between the rotor and the stator of gas to be expanded. This seal gas limits the flow rate of cold loss and reheats it before its mixture flows to the bearings sustaining the rotor;
on the other hand, a larger flow rate at ambient temperature, called bearing gases, is injected within the support bearings, where it is subjected to a generally sonic expansion.
The utility gases are then collected and sent to the residual gas network of the installation.
The invention has for its object to provide a process leading to important valorization of the gas utilized, and an increased output of the process with minimum loss of purified hydrogen.
To this end, the invention has for its object a process of the type recited above, characterized in that the turbine is supplied with gas leaving the bearings, after cooling this gas in the heat exchange line, and there is withdrawn a flow rate of hydrogen leak at the level of the labyrinthine seal of the turbine, under a pressure slightly less than the interstitial pressure Pi which prevails between the rotor and the stator of this turbine.
The process can comprise one or several of the following characteristics:
the supply of the turbine is completed by a flow of purified hydrogen expanded in an expansion valve;
the leakage flow is added, if desired after expansion in an expansion valve, to the residual gas at ambient temperature;
the leakage flow is added, after cooling and if desired expansion, to said expanded residual fraction.
The invention also has for its object an installation adapted to practice the process defined above. This installation, of the type comprising a heat exchange line, at least one phase separator at the cold end of this heat exchange line, a hydrogen expansion turbine with gas bearings, means to cause to circulate a current of impure hydrogen in one direction in the heat exchange line, means to expand to a low pressure the liquid phase from the phase separator, means to add hydrogen under low pressure to the residual fraction resulting from this expansion, and means to cause to circulate the mixture obtained under the low pressure in countercurrent with the impure hydrogen, is characterized in that the heat exchange line comprises cooling passages for the gas leaving the turbine bearings, these passages being connected to the input of the turbine at the cold end of the heat exchange line, the installation comprising means to withdraw a leakage flow of hydrogen from the labyrinthine seal of the turbine, under a pressure slightly less than the interstitial pressure P1 which prevails between the rotor and the stator of this turbine.